Diesel fuels are used widely in automotive transport largely due to their high fuel economy. However, one of the problems when such fuels are burned in internal combustion engines is the pollutants in the exhaust gases that are emitted into the environment. For instance, some of the most common pollutants in diesel exhausts are oxides of nitrogen (hereafter abbreviated as “NOx”), particulate matter (including inter alia soot, adsorbed hydrocarbons and sulfates), unburned hydrocarbons, and to a lesser extent carbon monoxide. Also, sulfur dioxide emissions from diesel fuel exhaust gases are becoming increasingly a problem due to their affinity with after-treatment devices designed to reduce NOx and particulate emissions, thereby adversely affecting the functioning efficiency. The oxides of sulfur have been reduced considerably by reducing the sulfur levels in the diesel itself through refining operations such as by hydrodesulfurization. However, further advances are required to meet increasingly demanding worldwide legislation for progressively lower diesel powered vehicle exhaust emissions, especially NOx and particulate matter. An established trade-off exists between the two pollutants, i.e. NOx and particulate matter, whereby an increase in one leads to a decrease in the other, for a given engine and operating conditions.
A typical example of such a scenario is U.S. Pat. No. 5,792,339 in which a diesel oil composition comprising 250-495 wppm sulfur, 5-8.6 wt. % of polynuclear aromatics (PNAs) and 10-23.9 wt. % total aromatics is disclosed. At the same time, further advances in sulfur-sensitive after-treatment technology have led to increasing demand for lower levels of sulfur in diesel fuels.
There are a variety of analytical techniques that have been reported for measurement of total aromatics and polynuclear aromatics. In the discussion and claims that follow, aromatics and PNAs are measured by high performance liquid chromatography (HPLC) as defined by test number IP 391/95, unless otherwise indicated. IP391/95 is described in “IP Standard Methods for Analysis and Testing of Petroleum & Related Products, and British Standard 2000 Parts, 58th edition, February, 1999. This publication is incorporated herein by reference. Boiling range distillation determinations were performed via gas chromatography according to ASTM D2887 providing the temperature at which 10% of the fuel was recovered (T10) and the temperature at which 95% of the fuel was recovered (T95).
Hydrodesulfurization processes that reduce PNAs typically reduce monocyclic aromatics as well as resulting in higher than desired hydrogen consumption. Legislation requiring reduced sulfur content is also anticipated. For example, proposed sulfur limits for distillate fuels to be marketed in the European Union for the year 2005 is 50 wppm or less. Further, the maximum allowable total aromatics level for California Air Resources Board (CARB) reference diesel and Swedish Class I diesel are 10 and 5 vol. %, respectively. Further, the CARB reference fuels allows no more than 1.4 vol. % polyaromatics (PNAs). In Europe, from the year 2000, a limit of polynuclear aromatic content in diesel fuel has been set at 11% by weight but no limit has been set for the total aromatic content (including monocyclic aromatics) of the fuel. Consequently, much work is presently being done in the hydrotreating art because of these proposed regulations.
Hydrotreating, or in the case of sulfur removal, hydrodesulfurization, is well known in the art and typically requires treating the petroleum streams with hydrogen in the presence of a supported catalyst at hydrotreating conditions. The catalyst is usually comprised of a Group VI metal with one or more Group VIII metals as promoters on a refractory support. Hydrotreating catalysts that are particularly suitable for hydrodesulfurization, as well as hydrodenitrogenation, generally contain molybdenum or tungsten as the Group VI metal on alumina support promoted with cobalt, nickel, iron, or a combination thereof as the Group VIII metal. Cobalt promoted molybdenum on alumina catalysts are most widely used when the limiting specifications are hydrodesulfurization, while nickel promoted molybdenum on alumina catalysts are the most widely used for hydrodenitrogenation, partial aromatic saturation, as well as hydrodesulfurization.
Much work is also being done to develop more active catalysts and to improve reaction vessel designs in order to meet the demand for more effective hydroprocessing processes. Various improved hardware configurations have been suggested. One such configuration is a co-current design where feedstock flows downwardly through successive catalyst beds and treat gas, which is typically a hydrogen-containing treat gas, also flows downwardly, co-current with the feedstock. Another configuration is a countercurrent design wherein the feedstock flows downwardly through successive catalyst beds counter to upflowing treat gas, which is typically a hydrogen-containing treat-gas. The downstream catalyst beds, relative to the flow of feed, can contain high performance, but otherwise more sulfur sensitive catalysts because the upflowing treat gas carries away heteroatom components, such as H2S and NH3, that are deleterious to sulfur and nitrogen sensitive catalysts.
Other process configurations include the use of multiple reaction stages, either in a single reaction vessel, or in separate reaction vessels. More sulfur sensitive catalysts can be used in the downstream stages as the level of heteroatom components becomes successively lower. European Patent Application 93200165.4 teaches such a two-stage hydrotreating process performed in a single reaction vessel.
Distillate fuel compositions are taught that meet some of the low emissions requirements. For example, U.S. Pat. No. 5,389,111 teaches a diesel fuel composition having an aromatics content in the range from about 13 to 20 wt. %, a cetane number from about 54 to 60, which cetane number and aromatics content being within a certain area defined in FIG. 1 of that patent. U.S. Pat. No. 5,389,112 teaches a low emissions diesel fuel composition having an aromatics content in the range of about 14.3 to 19.7 wt. %, a cetane number from about 53.4 to 60.8, which cetane number and aromatics content falls within a certain area of FIG. 1 of their patent.
While distillate fuel compositions exist that produce lower levels of emissions than years past, there is still a need in the art for fuels with ever lower emissions levels that are needed to meet the ever stricter environmental regulations.
It has now been found that by controlling the amount of sulfur, PNAs and total aromatics in the diesel fuel within specific limits, the amount of NOx and particulates emitted from exhausts can be synergistically reduced.